Adaptive pulse compressor

ABSTRACT

An adaptive pulse compressor is described, especially for use with ultrashort pulses, wherein the input pulses are modified in an iterative fashion, according to a feedback signal obtained from measurement of the output pulses. The feedback signal is programmed, by means of a programmable spatial light modulator, to allow for independent control of the individual spectral components of the incoming pulses, such that almost arbitrary phase functions can be realized to accomplish efficient compression. One of the main disadvantages associated with prior art pulse compressors, namely, the need for characterization of the uncompressed pulses, is thus eliminated. The compressor is thus capable of handling completely uncharacterized or from time varying sources.

FIELD OF THE INVENTION

The present invention relates to the field of compression of ultrashortoptical pulses.

BACKGROUND OF THE INVENTION

Since the invention of mode-locked lasers, considerable effort has beendirected towards the generation of ultrashort optical pulses. Noveltechniques for broad-band dispersion control now enable self-mode-lockedTi:Sapphire lasers to directly produce 6.5 femtosecond pulses.Compression of pulses down to sub-5 fs is achievable by treating theoutputs of such lasers with novel spectral broadening techniques inexternal pulse compressors. Efficient pulse compression generallyrequires characterization of the pulses. Grating-pair or prism-paircompressors are commonly used to compensate mainly for second orderdispersion, while a combination of these allows for simultaneouscompensation for the second and the third orders, as described, forinstance in an article by R. L. Fork, C. H. B. Cruz, P. C. Becker and C.V. Shank, entitled “Compression of optical pulses to six femtoseconds byusing cubic phase compensation” published in Optics Letters, Vol. 12, p.483 (1987). More recently, chirped dielectric mirrors, tailored toproduce negative group velocity dispersion over a wide spectrum, havebeen described by R. Szipöcs, K. Ferencz, C. H. Spielmann, and F.Krausz, in the article entitled “Chirped multilayer coating forbroadband dispersion control in femtosecond lasers” which appeared inOptics Letters, Vol. 19, p. 201 (1994). Such chirped mirrors have beenused to compress pulses down to durations of sub-5 fs, as described inthe article entitled “Compression of high-energy laser pulses below 5 fs.” by M. Nisoli, S. De Silvestri, O. Svelto, R. Szipöcs, K. Ferencz, Ch.Spielmann, S. Sartania, and F. Krausz, published in Optics Letters, Vol.22, p. 522 (1997).

However, in cases where the pulses are uncharacterized, or when thespectral phase cannot be approximated by the leading few terms of thecorresponding Taylor expansion, these techniques for pulse compressioncannot be used efficiently, since the spectral transfer function neededto form the desired output pulse cannot be calculated. A specificspectral transfer function corresponds to a specific complex input pulsespectrum. Consequently, compression of arbitrary uncharacterized pulsesdown to the minimum time-bandwidth product cannot be accomplished bythese prior art methods since the relative phases between the spectralcomponents of the input pulses are not known.

Furthermore, practical considerations limit the use of such techniquesto situations where the pulse source is substantially constant in time.Actual laser sources undergo slow variations in time, therefore severelylimiting the usefulness of these techniques for the compression ofultrafast pulses. As the speed of optical communication increases, thedisadvantages of currently available pulse compression technologiesbecome more and more felt. There therefore exists a critical need for afaster, more efficient, versatile pulse compressor, capable of handlingarbitrary optical pulses with durations of the order of femtoseconds.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved pulse compressor,which overcomes the disadvantages and drawbacks of existing ultrashortpulse compressors, especially with respect to their application topulses which are completely uncharacterized or partially characterized,or which originate from a laser source whose output varies with time. Animproved femtosecond pulse compressor according to the presentinvention, has been first described by applicants in an article entitled“Adaptive ultrashort pulse compression and shaping” published in OpticsCommunications, Vol. 138, pp. 345-348 (June 1997), and experimentalresults obtained with such a pulse compressor have been reported byapplicants in an article entitled “Adaptive femtosecond pulsecompression”, published in Optics Letters, Vol. 22, pp. 1793-1795 (Dec.1997). These articles, as well as the disclosures of all publicationsmentioned in this section and in the other sections of thespecification, and the disclosures of all documents cited in any ofthose publications, are hereby incorporated by reference.

The femtosecond pulse compressor according to the present invention usesan adaptive technique. The use of adaptive control for pulse shaping wasfirst suggested by R. S. Judson and H. Rabitz, in their article entitled“Teaching lasers to control molecules”, published in Physical ReviewLetters, Vol. 68, p. 1500 (1992), and later by A. M. Weiner in hisextensive review article entitled “Femtosecond optical pulse shaping andprocessing”, published in Progress in Quantum Electronics, Vol. 19, pp.161-237 (1995). In those articles, however, the authors suggest usingthe adaptive procedure for pulse shaping only, such pulse shaping beingused, for instance, in optimizing the yield of laser induced chemicalreactions. No suggestion was made anywhere in these articles to use anadaptive technique for pulse compression, for use, for instance, inoptical communication networks.

There is thus provided in accordance with a preferred embodiment of thepresent invention, an adaptive pulse compressor, especially for use withultrashort pulses, wherein the input pulses are modified in an iterativefashion, according to a feedback signal obtained from measurement of theoutput pulses. The feedback signal is derived from any measured propertyof the output pulse that increases with the intensity of the pulse as itis compressed. Any non-linear property may be used as the phenomenonfrom which to derive this feedback signal, such as the generation of asecond harmonic signal from the pulse by means of a non-linear crystal.This feedback signal is used with a suitable optimization algorithm tocontrol the spectral components of the incoming pulses, so as tomaximize the intensity of the output pulse, and hence to provide maximumcompression. The control is performed by means of a programmable spatiallight modulator, such that almost arbitrary phase functions can berealized to accomplish efficient compression. The adaptive femtosecondpulse compressor according to the present invention, thus removes one ofthe main disadvantages associated with prior art pulse compressors,namely, the need for characterization of the uncompressed pulses. Thus,the use of the adaptive technique in this invention, together with theability to form almost arbitrary spectral phase and/or amplitudefiltering, allows the efficient and versatile compression of arbitrarypulses.

Additional important benefits arise from the use of adaptive pulsecompression. For example, the adaptive scheme can be used not only tocompress pulses, but also to correct for slow variations of the lasersource. Another benefit arises from the fact that once the optimalfilter has been calculated as a result of the compression process, thisfilter essentially provides a measurement of the spectral phase of everycomponent of the original pulse. This data, together with the powerspectrum, allows for full characterization of the pulses.

There is further provided in accordance with another preferredembodiment of the present invention, an adaptive femtosecond pulsecompressor, including an adaptive optical pulse compressor forcompressing input pulses into output pulses of shorter duration, theinput pulses being modified by an iterative procedure according to afeedback signal obtained from a measurement of the output pulses.

In accordance with yet another preferred embodiment of the presentinvention, there is provided an adaptive optical pulse compressor asdescribed above, and wherein the input pulses are either uncharacterizedor partially characterized, or where the input pulses are time varying.

In accordance with a further preferred embodiment of the presentinvention, there is also provided an adaptive optical pulse compressoras described above, and wherein the iterative procedure uses anoptimization algorithm, which may preferably be a simulated annealingoptimization procedure.

In accordance with still another preferred embodiment of the presentinvention, there is provided an adaptive optical pulse compressor asdescribed above, and wherein the iterative procedure is an optimizationalgorithm with constraints as to the range of dispersion coefficients tobe used in the Taylor expansion expressing the input pulsecharacteristics.

There is further provided in accordance with yet another preferredembodiment of the present invention, an adaptive optical pulsecompressor as described above, and wherein the input pulses havedurations of the order of femtoseconds.

There is further provided in accordance with still another preferredembodiment of the present invention, an adaptive optical pulsecompressor for compressing input pulses into output pulses of shorterduration, including a programmable pulse shaper, optical apparatus forproviding a feedback signal dependent on the intensity of the outputpulses, and a computing system operative to control the programmablepulse shaper according to the feedback signal.

There is further provided in accordance with yet another preferredembodiment of the present invention, an adaptive optical pulsecompressor as described above, and wherein the programmable pulse shaperincludes an input dispersive device for spatially separating frequencycomponents of the input pulse, a programmable spatial light modulator, afocusing element for focusing the spatially separated frequencycomponents onto the programmable spatial light modulator, a focusingelement for collimating the spatially separated frequency componentsafter transmission through the programmable spatial light modulator, andan output dispersive device for recombining the spatially separatedfrequency components outputted from the programmable spatial lightmodulator.

There is provided in accordance with still a further preferredembodiment of the present invention, an adaptive optical pulsecompressor as described above, and wherein at least one of the input andoutput dispersive devices is a holographic grating, a ruled grating or aprism.

There is even further provided in accordance with a preferred embodimentof the present invention, an adaptive optical pulse compressor asdescribed above, and wherein at least one of the focusing elements is alens or a mirror, or wherein at least one of the dispersive devices isalso operative as a focusing element.

There is also provided in accordance with a further preferred embodimentof the present invention, an adaptive optical pulse compressor asdescribed above, and wherein the programmable spatial light modulator isoperative to control at least the phase of the individual spatiallyseparated frequency components of the input pulse.

In accordance with yet another preferred embodiment of the presentinvention, there is provided an adaptive optical pulse compressor asdescribed above, and wherein the feedback signal is a non-linearmeasurement of the output pulse.

In accordance with a further preferred embodiment of the presentinvention. there is also provided an adaptive optical pulse compressoras described above, and wherein the optical apparatus for providing afeedback signal dependent on the intensity of the output pulsescomprises a harmonic generator and a photo-detector.

In accordance with still another preferred embodiment of the presentinvention, there is provided an adaptive optical pulse compressor asdescribed above, and wherein the harmonic generator is a second harmonicgenerator, which may be a non-linear crystal.

There is further provided in accordance with yet another preferredembodiment of the present invention, an adaptive optical pulsecompressor as described above, and wherein the optical apparatus forproviding a feedback signal dependent on the intensity of the outputpulses comprises apparatus for providing a multi-photon fluorescenceoutput signal and a photo-detector.

There is further provided in accordance with still another preferredembodiment of the present invention, an adaptive optical pulsecompressor as described above, and wherein the multi-photon fluorescenceoutput signal is a two-photon fluorescence output signal.

There is provided in accordance with still a further preferredembodiment of the present invention, an adaptive optical pulsecompressor as described above, operative to correct for distortion inthe output pulses of a laser or a laser amplifier chain.

There is also provided in accordance with a further preferred embodimentof the present invention, an adaptive optical pulse compressor asdescribed above, operative to provide dispersion compensation in anoptical data communication channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings, in which:

FIG. 1 is a schematic view of an adaptive femtosecond pulse compressor,constructed according to the present invention, showing the componentparts of the system. A non-dispersive 4-f pulse shaper is used, and thefeedback signal is taken from the second harmonic signal of the outputpulse.

FIGS. 2A and 2B show interferometric autocorrelation traces of theuncompressed and compressed pulses. FIG. 2A shows the uncompressed 80 fspulses, as obtained directly from the Ti:Sapphire laser source. Thepower spectrum is shown in the inset. FIG. 2B shows the resultingcompressed pulses after 1000 iterations. The pulses are observed to becompressed down to 14 fs.

FIG. 3 is a plot of the second harmonic feedback signal in mV, as afunction of the iteration number, when a modified simulated annealingalgorithm is used for compression.

FIG. 4 shows an interferometric autocorrelation trace of the compressedpulses, obtained using a two-dimensional search algorithm for second andthird order dispersion coefficients only, after 1200 iterations. Thepulses are observed to be compressed down to 11 fs. The output pulsepower spectrum is shown in the inset.

FIG. 5 is a schematic drawing of a multi-stage laseroscillator/amplifier chain, utilizing an adaptive pulse compressoraccording to the present invention to precompensate for phase and/oramplitude distortion of the output pulses, introduced by the amplifierchain.

DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1 which is a schematic view of an adaptivefemtosecond pulse compressor, constructed and operative according to apreferred embodiment of the current invention. The pulse to becompressed 10 is inputted to a non-dispersive 4-f pulse shaper 12 of thetype described by A. M. Weiner, D. E. Leaird, J. S. Patel, and J. R.Wullert in their articles in Optics Letters, Vol. 15, p. 326 (1990), andin the IEEE Journal of Quantum Electronics, Vol. 28, p. 908 (1992). Thispulse shaper is comprised of a pair of dispersive elements 14, 15, and apair of focussing elements 16, 17, with a programmable one-dimensionalliquid crystal Spatial Light Modulator array 18 (Model SLM-256,manufactured by CRI Incorporated of Boston, Mass.) located at theFourier plane of the focal elements. This Spatial Light Modulator (SLM)has polarizers acting as half wave plates on either side of the liquidcrystal elements, in order to ensure the correct polarization of thelight for the most effective operation both of the gratings and of theliquid crystal element itself. In this preferred embodiment, thedispersive elements 14, 15 are thin holographic transmission gratingswith 400 lines/mm, and the focussing elements 16, 17 are achromatlenses, each having a focal length of 100 mm.

The operation of the 4-f pulse shaper is as follows. After passingthrough the first grating 14, the frequency components that make up theincident pulse are spread out into spatially different positions. Theinput lens 16 focuses each of these frequency components to its specificposition at the focal plane, where the SLM 18 is located. The SLM isoperative as an updatable filter for spectral manipulation of theincoming pulses. This SLM allows the independent control of the phaseand amplitude for each of its 128 pixels 20, thereby modifying theoutput pulse shape and temporal profile according to the desired pulseproperties. The width of each pixel 20 is 97 μm, the inter-pixel gap is3 μm, while the spot size at the focal plane is about 80 μm. The outputlens 17 and output grating 15 then recombine each of the separatefrequency components to produce the output pulse 19.

Though the above embodiment uses thin holographic transmission gratings14, 15 at the input and output of the pulse shaper to spatially disperseand recombine the various frequency components of the pulses, it shouldbe pointed out that any suitable dispersive elements can be used, suchas ruled gratings, reflective gratings, or prisms, or combinations ofthem.

Furthermore, the function of the lenses 16, 17 in defining the systemFourier plane at which the SLM 18 is located, can be fulfilled by anyother element having positive focusing power, such as a concave mirror.

The 4-f pulse shaper used in the above described embodiment is only oneof the possible pulse shapers that can be used in the adaptive pulsecompressor of the present invention. According to further preferredembodiments of the present invention, any suitable dynamic programmablepulse compressor may be used, such as that which uses an acousto-opticcell as the spatial light modulator, as described by C. W. Hillegas, J.X. Tull, D. Goswami, D. Strickland and W. S. Warren in their article inOptics Letters, Vol. 19, p. 737 (1994) entitled “Femtosecond laser pulseshaping by use of microsecond radio frequency pulses”.

A sample of the output pulse 19, for use as the feedback signal, isobtained by means of a beam splitter 22. Though the beam splitter shownin FIG. 1 uses the reflected component as the sample beam, and thetransmitted signal as the output beam, it is understood that theinvention will operate just as well if these roles are reversed,provided that the correct signal levels for each function aremaintained. As an alternative to a beam splitter, it is possible to usea gated mirror shutter, whereby the mirror reflects all of the signalinto the feedback path while the iteration is in progress, and onachieving the desired compression level, the mirror is moved to analternative position, whereby the compressed pulse is outputted.

The sample is focused using a lens 24 of focal length 50 mm, onto a 100μm thick frequency-doubling BBO non-linear crystal 26. The secondharmonic signal generated within the frequency doubler is detected by aphotomultiplier 28, from which the electronic signal is fed to a lock-inamplifier 30. The phase detected signal is inputted by means of a cable34 to a computer 32, which is programmed to calculate the spectralfilter and to update the SLM status accordingly.

In the general case of pulse compression, where the input pulses arecompletely uncharacterized, efficient compression is accomplished by useof a modified simulated annealing optimization procedure for maximizingsuch a single-value feedback signal. According to this invention, anyoptimization algorithm can be used to accomplish convergence of thepulse to its optimum temporal shape. Various specific algorithms thathave been proposed, such as the genetic algorithms or the evolutionarystrategies proposed by Baumert et al. in their article “Femtosecondpulse shaping by an evolutionary algorithm with feedback”, published inApplied Physics B, Vol. 63, pp. 779-782 (Dec. 1997). The differencesbetween the various algorithms proposed only causes the process toconverge after more or less iterations, depending on the efficiency ofthe algorithm for the specific process.

According to another preferred embodiment of the present invention,phase-only filtering may be used, in order to accomplish efficientspectral manipulation. At iteration i of the algorithm, thetime-integrated second harmonic signal C_(i) of the output pulses isgiven by C_(i)=∫I_(i) ^((2ω))(t) dt, where I_(i) ^((2ω))(t) is thesecond harmonic intensity of the output pulses. Since phase-onlyfiltering involves no energy loss, and due to the nonlinear processinvolved in the generation of the second harmonic pulses, high levelfeedback signals correspond to high peak pulses of short duration.Although this feedback signal cannot provide full information about theshape of the pulses, it has been verified numerically by D. Meshulach etal. in op. cit., that for a variety of pulses, such a feedback signal iseffective for efficient compression. In order to carry out the adaptivecompression process, a modified simulated annealing algorithm, whichinvolves maximization of this feedback signal, may be used. At eachiteration, a random change to the spectral filter is applied, thefeedback signal is measured, and a decision is taken whether to acceptthis change.

The above embodiment has been described using the second harmonic signalas the feedback signal. According to further preferred embodiments ofthe present invention, the feedback signal can be taken from anyphysical phenomenon whose output is a function of the pulse intensity,such as higher order harmonics of the output pulse, or multi-photonfluorescence effects of the output pulse.

Since the adaptive compression used in the pulse compressor according tothe present invention is accomplished by maximization of the feedbacksignal, this method accounts not only for the spectral phase of theinput pulses, but also for any other phase distortions induced in thecompressor, such as residual dispersion of the shaper, the mirrors andthe SLM itself Once an optimal phase function has been found, itprovides an indirect measurement of all of these phase distortions.Therefore, although the compressor is operative for pulses having smoothspectral phase variations, such as those emerging directly from lasers,based on the performance of programmable pulse shapers, such as thosedescribed by A. M. Weiner in op. cit., by D. Meshulach et al. in op.cit., and by M. M. Wefers, and K. A. Nelson, in Optics Letters, Vol. 20,p. 1047ff (1995), it is believed that the technique also has thepotential to compensate for almost arbitrary spectral phase distortions.

On the other hand, we note that the SLM imposes several limitations onthe overall performance. Since spectral manipulation is accomplished bycontrol over 128 discrete pixels, continuous filters cannot be realized.In addition, some modulation of the power spectrum is present, due toresidual amplitude modulation in the SLM. Consequently, true phasemodulation is not achieved. These difficulties, associated with thepixelization of the filter and the residual amplitude modulation, setlimitations on the overall performance of the adaptive pulse compressoraccording to the present invention, as is shown in the measurementsdiscussed hereinunder.

In order to demonstrate the operation of an adaptive pulse compressoraccording to the present invention, some measurements were made on anexperimental compressor constructed and operative according to the abovementioned preferred embodiment. The femtosecond pulse source used was amode-locked Ti:Sapphire laser, where intracavity dispersion iscompensated for by a prism-pair arrangement, followed by a secondprism-pair, external to the cavity, as described by F. Krausz, Ch.Spielmann, T. Brabec, E. Wintner, and A. J. Schmidt, in the article“Generation of 33 fs pulses from a solid state laser” published inOptics Letters, Vol. 17, p. 204 (1992). This laser normally producespulses of sub-20 fs duration.

To exploit the full potential of adaptive compression of pulses obtaineddirectly from a laser, the laser was adjusted to produce a widespectrum, as shown in the inset of FIG. 2A. In this case, as isgenerally the case for mode-locked Ti:Sapphire lasers with intracavityprism-pair arrangements, the pulses obtained were far from transformlimited. FIG. 2A illustrates an interferometric autocorrelationmeasurement of the pulses from the laser, and implies strongly chirpedpulses of 80 fs FWHM. The inset graph shows the power spectrum of thepulse.

On closing the feedback loop, adaptive compression is obtained, and theincrease of the second-harmonic signal is monitored as the processprogresses. Interferometric autocorrelation measurements of thecompressed pulses are shown in FIG. 2B. In this case, the optimizationwas performed using a modified simulated annealing algorithm with 1000iterations. On comparing this result with the measurement of theuncompressed pulse shown in FIG. 2A, it is observed that the 80 fs inputpulses are compressed down to 14 fs. The difference in the time scalebetween the two figures should be noted.

FIG. 3 shows the feedback signal in each iteration used to obtain theresult shown in FIG. 2B. The second harmonic power increased by a factorof about 3 as a result of the optimization process. Since the pulseswere compressed by a factor of about 5.7, a similar increase in thesecond harmonic signal would have been expected. This difference may beattributed to some energy loss in the SLM during the process, and tosome residual negative dispersion of the shaper. The rapid variations inthe feedback signal are not noise, but are due to the search procedureof the algorithm. In other demonstrations, pulses were compressed downto 17 fs and 13 fs after 500 and 2000 iterations, respectively. Thissuggests a trade-off between the number of iterations and the durationof the resulting compressed pulses. It is noted that a relatively smallnumber of iterations is needed for optimization of 128 independent phaseterms, this form of convergence being typical for the compressoraccording to the present invention.

The modified simulated annealing algorithm proposed does not make use ofany information about the uncompressed pulses, but simply maximizes thefeedback signal by any suitable optimization procedure. This is whatgives the pulse compressor according to the present invention its uniqueadvantage, namely of being able to operate on any uncharacterized inputpulse. If, however, some information on the input pulses is available,it can be used to improve the optimization procedure. For example, thiscould be the case where the pulses exhibit dispersion which can beapproximated by the leading terms of the corresponding Taylor expansion.In this case, an optimization algorithm for adaptive compression basedon a search in the two-dimensional space of second and third orderdispersion coefficients can be used to improve the results. Therefore,according to another preferred embodiment of the present invention, thepulse compressor may utilize an optimization algorithm with built inconstraints as to the range of dispersion coefficients to be used in theTaylor expansion expressing the input pulse characteristics.

Since most of the dispersion in an arbitrary input pulse usually arisesfrom the first 3 or 4 terms of the Taylor expansion of the dispersionrelationship, it is generally sufficient to use no more than these firstfew terms in the optimization algorithm.

FIG. 4 shows an interferometric autocorrelation measurement of thecompressed pulses obtained from those in FIG. 2A, by using such aconstrained algorithm. The algorithm used was a two-dimensional searchalgorithm for second and third order dispersion coefficients, and theresult is shown after 1200 iterations. From FIG. 4, it can be calculatedthat the pulses are compressed down to 11 fs. A direct Fouriertransformation of the input spectrum in the absence of any phasemodulation indicates that the spectrum can support a pulse as short as 9fs. This result suggests that the input laser pulses are indeed chirpedprimarily by second and third order terms of the spectral phase. Theinset graph shows the output power spectrum of the pulse.

An inspection of the inset graph of FIG. 4 demonstrates one of thelimitations on the overall performance of the adaptive pulse compressorconstructed, as discussed hereinabove. Some modulation of the powerspectrum is observed, due to residual amplitude modulation in the SLM.Consequently, true phase modulation was not achieved. This is not,however, a fundamental limitation of the performance of the compressor.

The adaptive pulse compressor according to the present invention canalso be used even when the pulse source undergoes slow variations intime, if the period of change is long compared with the time required toperform a complete compression, as discussed above. It is also possibleto provide features in the optimization algorithm which detects suddenchanges in the pulse form of the laser output, and to compensate forthem in the optimization procedure.

There are a number of applications for the adaptive pulse compressor ofthe present invention. All of these applications and others, arepossible only because the pulse compressor of the present inventionoperates without the need to know the form of the pulse beingcompressed. One such application is the improvement of the spectralcharacteristics of lasers, to correct for phase distortions in theoutput pulses. The narrowest pulse will give the transform limited pulseshape, and the addition of a pulse compressor according to the presentinvention to the output of a short-pulse laser will enable the laser toachieve such pulse improvement.

A further and important application is in the field of opticalcommunication, in order to provide dispersion compensation intransmission channels. As a result of the group-velocity dispersion andthe polarization mode dispersion present in the transmission link, thepulses of data become distorted, and hence widen in the temporal frame.The adaptive pulse compressor of the present invention is able torestore the original form of the pulse, without the need to know theform of the distortion.

Another potential application arises in the production of high powerultrashort laser pulses. FIG. 5 illustrates schematically such a system.Low power, ultrashort laser pulses 40 are produced by a laser oscillator41. These pulses are amplified by means of an amplifier chain 42, whichconsists of one or more stages of individual amplifiers 44. Since theamplifiers are not perfectly linear, each imparts some level of phaseand/or amplitude distortion to its input pulse, as a result of which,the pulse broadens. The high power amplified output pulse 45 can berestored to a shape close to that of the oscillator output, byprecompensating the input pulse to the amplifier chain by means of anadaptive pulse compressor 46, according to the present invention. Thepulse compressor uses a sample of the final output pulse as the feedbacksignal 48 which is optimized by the algorithm used.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

We claim:
 1. An adaptive optical pulse compressor, comprising: an inputport for receiving an input pulse; an output port for delivering anoutput pulse, said output pulse being obtained by compression of saidinput pulse; an optical pulse measurement device for determining theintensity of said output pulse; and a pulse shaper for modifying saidinput pulse by means of an iterative procedure, according to a feedbacksignal obtained from said intensity of said output pulse; wherein saiditerative procedure is an optimization algorithm with constraints as tothe range of dispersion coefficients to be used in the Taylor expansionexpressing the input pulse characteristics.
 2. An adaptive optical pulsecompressor, comprising: an input port for receiving an input pulsehaving a duration of the order of femtoseconds; an output port fordelivering an output pulse, said output pulse being obtained bycompression of said input pulse; an optical pulse measurement device fordetermining the intensity of said output pulse; and a pulse shaper formodifying said input pulse by means of an iterative procedure, accordingto a feedback signal obtained from said intensity of said output pulse.3. An adaptive optical pulse compressor for compressing input pulsesinto output pulses of shorter duration, comprising; a programmable pulseshaper; optical apparatus for providing a feedback signal dependent onthe intensity of said output pulses; and a computing system operative tocontrol the programmable pulse shaper according to said feedback signal;wherein said programmable pulse shaper comprises: an input dispersivedevice for spatially separating frequency components of said inputpulse; a programmable spatial light modulator; a focusing element forfocusing said spatially separated frequency components onto saidprogrammable spatial light modulator; a focusing element for collimatingsaid spatially separated frequency components after transmission throughsaid programmable spatial light modulator; and an output dispersivedevice for recombining said spatially separated frequency componentsoutputted from said programmable spatial light modulator.
 4. An adaptiveoptical pulse compressor according to claim 3, and wherein at least oneof said input and output dispersive devices is a holographic grating. 5.An adaptive optical pulse compressor according to claim 3, and whereinat least one of said input and output dispersive devices is a ruledgrating.
 6. An adaptive optical pulse compressor according to claim 3,and wherein at least one of said input and output dispersive devices isa prism.
 7. An, adaptive optical pulse compressor according to claim 3,and wherein at least one of said focusing elements is a lens.
 8. Anadaptive optical pulse compressor according to claim 3, and wherein atleast one of said focusing elements is a mirror.
 9. An adaptive opticalpulse compressor according to claim 3, and wherein at least one of saiddispersive devices is also operative as a focusing element.
 10. Anadaptive optical pulse compressor according to claim 3, and wherein saidprogrammable spatial light modulator is operative to control at leastthe phase of said individual spatially separated frequency components ofsaid input pulse.
 11. An adaptive optical pulse compressor according toclaim 3, and wherein said feedback signal is a non-linear measurement ofthe output pulse.
 12. An adaptive optical pulse compressor according toclaim 3, and wherein said optical apparatus for providing a feedbacksignal dependent on the intensity of said output pulses comprises aharmonic generator and a photo-detector.
 13. An adaptive optical pulsecompressor according to claim 12, and wherein said harmonic generator isa second harmonic generator.
 14. An adaptive optical pulse compressoraccording to claim 13, and wherein said second harmonic generator is anon-linear crystal.
 15. An adaptive optical pulse compressor accordingto claim 3, and wherein said optical apparatus for providing a feedbacksignal dependent on the intensity of said output pulses comprisesapparatus for providing a multi-photon fluorescence output signal and aphoto-detector.
 16. An adaptive optical pulse compressor according toclaim 15, and wherein said multi-photon fluorescence output signal is atwo-photon fluorescence output signal.
 17. An adaptive optical pulsecompressor according to claim 3, operative to correct for distortion inthe output pulses of a laser.
 18. An adaptive optical pulse compressoraccording to claim 3, operative to provide dispersion compensation in anoptical data communication channel.
 19. An adaptive optical pulsecompressor according to claim 3, operative to correct for distortion ina laser amplifier chain.